Many airlines allow you to send a mobile boarding pass to your phone, but just because you load it in your device's browser doesn't mean it will still be there when you get to the security checkpoint and/or gate. The solution? Redditor thewilyone suggests taking a screenshot:

I use the Delta App all the time when traveling and I hate that it had to reconnect every time it needs to access the pass. Today I had to wait 5 minutes and let a ton of people pass me in line waiting for it to refresh. A friend told me this trick and it make so much sense.

I do this all the time and it saves me from so much potential frustration. Screenshots take so much less effort than trying to load your boarding pass when you're waiting in line. There's really no better way to save your boarding pass.